Speaking as an iOS developer who's had a hand in hiring, it's the same as the "slightly broken Apache and Tomcat" scenario someone posted for sysadmins. We ship people a slightly broken app with a few common-yet-googleable problems, then ask them to build a tiny thing on top of that and ship it back to us.

The ability to solve common problems acts as a good proxy for experience, If they can't google how to fix a retain cycle or a broken constraint, we know they probably don't know what they're talking about.

The ability to build a good, small system on top of the broken app acts as a good proxy for development talent. If they can't easily set up a dead-simple delegate to do an asynchronous web call, they can't hack as a mobile developer.

Most importantly, if they don't care enough about the job to take two hours to whip up a super-simple app, then we don't have to waste our time interviewing them.